Device for checking the thickness and the cohesion of the interface of a duplex tube

ABSTRACT

The duplex tube (1) comprises a tubular core (2) and a cladding or covering layer (3) made from an alloy, the base metal of which is identical to the base metal of the alloy constituting the tubular core (2). Ultrasonic waves at normal incidence are emitted into the thickness of the covering (3) and of the core (2) of the tube (1), the ultrasonic waves reflected by the inner and outer surfaces of the tube, by its interface (4) and by any flaws in cohesion at the interface (4) are collected, the propagation times of the ultrasonic waves in the thickness of the tube (1) are measured, the amplitude and the shape of the reflected waves is determined, the tube (1), from its outer surface, is subjected to a magnetic induction created by a multi-frequency sinusoidal current, measurements of the phase and/or amplitude of the currents induced in the tube (1) are made, the thickness of the covering layer (3) is deduced therefrom, the total thickness of the tube (1) is calculated from the measurements of the propagation times of the ultrasonic waves and of the thickness of the covering layer (3), and the cohesion of the tube at its interface (4) is determined by analyzing the amplitude and the shape of the ultrasonic waves reflected by the interface (4) or transmitted by the covering layer (3).

This is a Division of application Ser. No. 07/711,997, filed Jun. 7,1991, now U.S. Pat. No. 5,225,148.

FIELD OF THE INVENTION

The invention relates to a device for checking the thickness and thecohesion of the interface of a duplex tube, and in particular of azirconium alloy duplex tube used as a jacket element for a fuel rod ofan assembly for a water-cooled nuclear reactor.

BACKGROUND OF THE INVENTION

The fuel assemblies of water-cooled nuclear reactors, and in particularof pressurized-water nuclear reactors, comprise a framework into whichare introduced fuel rods consisting of a Jacket enclosing a nuclearcombustible material such as uranium or plutonium oxide in the form ofsintered pellets.

The Jacket made from a zirconium alloy tube must have a good resistanceto corrosion under the effect of the primary fluid circulating incontact with the outer surface of the Jacket.

In order to constitute the Jacket of the fuel rods of the assemblies ofwater-cooled reactors, use is usually made of a zirconium-based alloycontaining mainly tin and iron.

In order to improve the corrosion stability under irradiation of theJackets of fuel rods in the operating environment of the nuclearreactor, and thus to increase the lifetime of the fuel assemblies in thecore, modifications or adjustments have been proposed to the compositionof these zirconium alloys, or alternatively it has been proposed toreplace these alloys containing tin, iron and chromium with alloyscontaining other elements such as vanadium, niobium or copper.

It has also been proposed, for example in EP-A-0,212,351, to produce theJacket in the form of a duplex tube comprising a tubular inner core madefrom a zirconium alloy of a conventional type such as that describedabove, and a surface layer consisting of a cladding or a coveringimproving the corrosion stability of the Jacket.

The zirconiumalloy constituting the cladding or covering layer differsfrom the alloy constituting the core of the tube and contains iron andat least one of the elements vanadium, platinum and copper. This surfacelayer, the thickness of which represents 5 to 20% of the total thicknessof the wall of the jacket, can be produced by extrusion of a billetconsisting of an inner tube made from zirconiumalloy of a conventionalcomposition over which is fitted an outer tube having the composition ofa surface layer.

The jacket is then rolled in a pilgrim step rolling mill to its finaldiameter.

More recently, there has been proposed in FR-A-89-00761 filed Jointly bythe companies FRAMATOME, COGEMA, CEZUS and ZIRCOTUBE, a duplex tube, thesurface layer of which, having a thickness lying between 10 and 25% ofthe total thickness of the wall of the Jacket, consists of azirconium-based alloy containing tin, iron and niobium or vanadium. Thetubular core of the duplex tube can be made from a conventionalzirconium alloy in the case of the manufacture of the Jackets for fuelrods, or from a zirconiumbased alloy containing mainly niobium as thealloying element.

In all cases, it is necessary to ensure the perfect quality of theduplex tubes which are intended to constitute Jackets for fuel rods, inparticular in terms of the diameter of the tube, the total thickness ofthe Jackets, the thickness of the outer cladding layer and the cohesionof the interface zone between the cladding layer and the core of thetube.

Checks must be carried out at the factory on very large quantities oftubes, the diameter of which is very small as compared to the length.

The checking of the diameter and the total thickness of the jacket canbe carried out by using a conventional technique consisting in measuringthe distance in the propagation times of pulse-shaped ultrasonic waveswhich are reflected by the outer surface and by the inner surface of thetube.

This ultrasonic checking and measuring technique, known under the nameof the "pulse-echo" technique, may be adapted in order to take accountof the cladding layer in the calculation of the total thickness of thejacket.

It has also been proposed to use a technique using ultrasonic waves inorder to check the thickness of the cladding of a duplex tube based onzirconium alloy.

This technique, described in FR-A-2,629,586 filed in the name of thecompany CEZUS, employs an ultrasonic-wave check adapted to themeasurement of a layer of small thickness, the acoustic properties ofwhich are very similar to those of the core of the tube of greaterthickness.

This improved technique does not, however, permit the measurement ofcladding thicknesses of less than 0.4 mm, inasmuch as the industrialimplementation of the method under satisfactory conditions requires theuse of ultrasonic waves whose frequency does not exceed 20 MHz.

In the case of-a cladding layer whose thickness lies between 80 and 100μm, which corresponds to the conditions encountered most commonly in thecase of duplex tubes used as Jacket material, it would be necessary toemploy ultrasonic waves at very high frequencies (for example of theorder of 100 MHz), which makes it extremely difficult to apply themethod in an industrial context.

Furthermore, in the case of jackets for fuel rods, the cladding layerand the tubular core of the duplex tube consist of very slightly alloyedzirconiumbased alloys which have very similar acoustic properties, withthe result that the coefficient of reflection of the acoustic waves atthe cladding/core interface is very small (generally less than 2%). Theinterface echo is then very small and becomes drowned out in theacoustic and electronic noise of the ultrasonic signal.

A measurement method and apparatus have been proposed in FR-A-2,534,015which make it possible to determine the thickness of a zirconiumcovering on a zirconium-alloy tube, employing the analysis and themeasurement of currents induced in the cladding layer of the duplextube, by magnetic induction, using an exciting current the frequency ofwhich is selected as a function of the nominal thickness of the claddingor covering layer of the tube.

The frequency selected and the processing of the signals correspondingto the induced currents likewise make it possible to eliminate, to acertain degree, the measurement errors resulting from a variation in thewidth of the air gap between the exciting coil and the wall of the tube.

This technique, which is relatively complex to implement, does not makeit possible, however, to compensate for the variations in theconductivity of the material constituting the core of the tube and thevariations in the conductivity of the material constituting thecladding.

Furthermore, this technique does not make it possible to checkindependently the total thickness of the tube and the cohesion of theinterface zone between the cladding or covering layer of the tube andthe tubular core.

SUMMARY OF THE INVENTION

The object of the invention is to provide a device for checking thethickness and the cohesion of the interface of a duplex tube comprisinga tubular core made from an alloy such as a zirconium alloy and coveredwith a covering or cladding layer made from an alloy the base metal ofwhich is identical to the base metal of the alloy constituting thetubular core, this device making it possible to check the geometricaldimensions of the duplex tube and, in particular, its total thickness,and the thickness of the covering and cladding layer, and to detectflaws in cohesion at the interface between the covering or claddinglayer and the tubular core.

To this end, for various measuring and checking zones, around thecircumference or along the length of the tube, the following operationsare carried out continuously or discontinuously:

ultrasonic waves are emitted in such a way that these waves arepropagated in the covering and in the core of the tube in substantiallyradial directions,

the ultrasonic waves reflected by the inner and outer surfaces of thetube, by its interface between the core and the covering and by anyflaws in cohesion at the interface, or transmitted by the covering orcladding layer, are collected,

the propagation time of the ultrasonic waves in the thickness of thetube is measured,

the amplitude and shape of the reflected waves is determined,

the tube is subjected, from its outer surface, to a magnetic inductioncreated by a multi-frequency sinusoidal current,

measurements are taken of the phase and/or amplitude of the currentsinduced in the tube, termed Foucault currents,

the thickness of the covering layer is deduced therefrom,

the total thickness of the tube is calculated from the measurements ofthe propagation times of the ultrasonic waves and of the thickness ofthe covering layer, and

the cohesion of the tube at its interface is determined by analyzing theamplitude and the shape of the ultrasonic waves reflected by theinterface or transmitted by the covering or cladding layer.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to make the invention readily comprehensible, severalembodiments of the invention will now be described by way of example andwith reference to the accompanying drawings.

FIG. 1 is an exploded perspective view of part of a duplex tube used asa Jacket element for a fuel rod.

FIG. 2A is a sectional view of the wall of a duplex tube showingschematically the implementation of a method for measuring thicknessfrom the propagation times of ultrasonic waves in the wall of the tube.

FIG. 2B is a diagram giving the amplitude of the ultrasonic wavesreflected by the walls of the tube shown in FIG. 2A as a function oftime.

FIGS. 3A, 3B and 3C show three alternative embodiments of a device formeasuring, by Foucault currents, the thickness of the covering orcladding layer of a duplex tube.

FIGS. 4A and 4B, 5A and 5B and 6A and 6B respectively are views similarto FIGS. 2A and 2B showing the implementation of the method fordetecting flaws in cohesion at the interface between the covering orcladding layer and the core of a duplex tube by an ultrasonic technique,according to three known alternatives.

FIGS. 7A and 7B, and FIGS. 8A and 8B are views similar to FIGS. 2A and2B respectively showing the implementation of a method for detectingflaws at the interface between a covering or cladding layer and themetallic core of a duplex tube by an ultrasonic transmission technique,according to the invention.

FIGS. 7A and 7B relate to a zone of a tube having no interface flaws.

FIGS. 8A and 8B relate to a zone of a tube having an interface flaw andto its detection by the transmission of ultrasonic waves.

FIG. 9 is a perspective view of a device for ultrasonic detection of theflaws of the interface of a duplex tube.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, a duplex tube 1 comprises a tubular core 2 made froma zirconium alloy covered externally by a cladding layer 3 made from asecond zirconium alloy, the composition of which differs from thecomposition of the alloy constituting the core 2.

The zirconium alloys constituting the core 2 and the cladding layer 3 ofthe duplex tube 1 are low-alloy zirconium alloys in which the content ofalloying elements is less than 1% by weight for each of these elements.

The tubular core 2 and the cladding layer 3 therefore have acousticproperties which are extremely similar to each other. Furthermore, thecovering or cladding layer 3 has a small thickness, generally between 60and 80 μm, the metallic core 2 itself having a thickness slightly lessthan 600 μm.

A duplex tube such as that shown in FIG. 1, used as a jacket for a fuelrod of a pressurized-water nuclear reactor assembly, generally has anexternal diameter of the order of 10 mm and a length of the order of 4m.

In FIG. 2A, the wall of a duplex tube such as that shown in FIG. 1 hasbeen shown in .section, comprising a tubular core 2 covered by acladding and covering layer joined to the metallic core along acylindrical interface surface 4.

In order to measure the total thickness of the wall of the Jacketconsisting of the core 2 and the cladding layer 3, an ultrasonictransducer 5 is used which emits an ultrasonic-wave beam in thedirection of the outer surface of the duplex tube, consisting of theouter surface of the cladding layer 3.

The tube 1 is immersed in a coupling medium consisting of a liquidpermitting the transmission of the ultrasonic waves emitted by thetransducer 5.

Part of the ultrasonic-wave beam 6 is reflected by the outer surface ofthe duplex tube in the form of a beam 6' which is collected by thetransducer 5 and converted into an electrical signal which istransmitted to a processing unit 7.

The corresponding echo 8 can be displayed on an oscillogram giving animage of its amplitude and of its position against a time scale.

The ultrasonic beam 6a transmitted through the wall of the duplex tubeis reflected, in the form of a beam 6'a, by the inner surface of thecore 2 of the duplex tube.

The ultrasonic beam 6'a is collected by the transducer 5 which convertsit into an electrical signal and enables it, by virtue of the processingmodule 7, to be displayed on the oscillogram in FIG. 2B in the form ofan echo signal 8a.

The time lag between the signal 8 and the signal 8a corresponds to twicethe time δT taken for the ultrasonic waves to travel through the wall ofthe tube 1.

It is possible to obtain an approximate value for the total thickness e₈of the jacket corresponding to the thickness of the wall of the duplextube by assuming that the speeds of propagation of the ultrasonic wavesin the metallic core of the Jacket and in the cladding layer areidentical.

This method of determination is only approximate, inasfar as the speedof propagation V_(p) of the longitudinal ultrasonic waves in thecladding material is not identical to the speed of propagation V_(a) ofthe ultrasonic waves in the material constituting the core of the duplextube.

On the other hand, the method of measuring directly the propagation timeof ultrasonic waves does not permit the measurement of the thickness ofthe cladding layer e_(p), the coefficient of reflection of the acousticwaves at the interface 4 between the cladding layer 3 and the core 2being very small (generally less than 2%), because the acousticproperties of the materials constituting the cladding layer and the coreare extremely similar to each other.

Furthermore, the cladding layer has a small thickness as compared withthe total thickness of the wall, with the result that the differences inthe propagation times to be taken into account are themselves verysmall.

In FIGS. 3A, 3B and 3C, three different embodiments of aFoucault-current device have been shown, permitting the measurement ofthe thickness of an outer cladding layer of a duplex tube 1 consistingof a metallic core covered with a cladding layer, the metallic core andthe cladding layer consisting of two zirconiumalloys containing verysmall quantities of alloying elements.

Small variations in alloying measurements in lowalloy alloys can giverise to very considerable variations in the electrical conductivity ofthese alloys.

For example, in the case of zircaloy, which is a zirconium alloycontaining tin, a variation of 1% in the tin content gives rise to avariation in conductivity of the order of 50%.

Such variations make it possible to apply the technique of inducedcurrents or Foucault currents in order to check the thickness of acladding layer whose composition differs from that of the metallic corecovered by the cladding layer.

It is possible to use, as shown in FIG. 3A, a coil 10 comprising acertain number of turns surrounding the tube 1.

The coil is supplied by a multi-frequency sinusoidal exciting currentvia a current source 11 connected to its terminals. The electricalsignals corresponding to the induced currents are processed by a aprocessing unit 12.

In the case of this first embodiment of the device for measurement byFoucault currents, in fact the mean value of the thickness of thecladding is measured, which incorporates the possible variations inthickness at the circumference of the tube, or circumferentialvariations. The variations in thickness over the length of the coil 10,or axial variations, are likewise incorporated.

According to this principle, the measurement is also sensitive to thecentering of the tube within the coil constituting the Foucault-currentprobe, so that this centering, even if carried out optimally, is likelyto reduce the accuracy of the measurement.

A second measurement technique such as that shown in FIG. 3B consists inusing a coil 14, the axis of which has a radial direction with respectto the tube 1.

The excitation of the coil by a multi-frequency sinusoidal current, byvirtue of a current source 11', and the processing of the signalscorresponding to the induced currents by a processing unit 12', arecarried out in the same way as in the case of the measurement deviceshown in FIG. 3A.

The device such as that shown in FIG. 3B makes it possible to carry outa localized measurement of the thickness of the cladding of the tube 1.

As shown in FIG. 3C, it is also possible to use a plurality of coils 15similar to the coil 14 shown in FIG. 3B and fixed on a common support16, so that the coils 15, the axes of which extend radially with respectto the tube 1, are arranged about the tube in regularly distributedcircumferential positions.

It is thus possible to carry out simultaneously thickness measurementsat various points distributed about the circumference of the tube.

It is also clear that it is possible to sweep the surface of the tube,for example by displacing the tube axially with respect to theFoucault-current probe, as shown by the arrow 13 in FIG. 3A.

The frequency of the sinusoidal exciting signal, and the dimensions ofthe windings (diameter and height) are determined so as to optimize thesensitivity of the measurements to the variations in thickness of thecladding and to minimize the variations of the measurement signalscaused by variations in the distance between the coil and the surface ofthe tube, constituting an air gap.

This air-gap or lift-off effect can be considerably reduced by anappropriate choice of the frequency, as indicated in FR-A-2,534,015.

In order to improve the quality of the measurement and, in particular,in order to take into account possible variations in electricalconductivity of the alloys constituting the core and the cladding of thetubes, this electrical conductivity being very sensitive to thecomposition of the alloys, it is possible to use, in addition to themain exciting frequency as defined above, one or more auxiliaryfrequencies intended to compensate for variations in composition on agiven tube or within a given batch of tubes or within a given castingoperation.

The invention is therefore characterized by the use of a multi-frequencysinusoidal exciting signal having a main frequency and secondaryfrequencies.

It is possible, in particular, to use a second frequency which issensitive to the mean variation in conductivity of the alloysconstituting the core and the cladding, this second frequency beinginsensitive, or having a very low sensitivity, to the variations inthickness of the core and of the cladding.

It is also possible to use two auxiliary frequencies, one of which issensitive to the variation in conductivity of the base materialconstituting the core while at the same time being very slightlysensitive to variations in conductivity of the cladding and tovariations in thickness of the core and of the cladding, and the otherof which is sensitive only to variations in conductivity of thecladding.

It is also possible to use a supplementary frequency to carry outmeasurements and compensations of the lift-off effect.

The probe is excited simultaneously by each of the sinusoidal signalshaving the frequencies determined in the manner described above, and thephase-measurement and amplitude-measurement signals corresponding toeach of the sinusoidal signals of determined frequency are digitized andprocessed, as indicated above, by a processing module and bydata-processing means which make it possible to deduce from thesesignals the value of the thickness of the cladding.

The measurement of the thickness of the cladding is obtained either byanalysis of the phase of the signal corresponding to the Foucaultcurrents, this method having the advantage of being less sensitive tothe variations in lift-off, or by combined analysis of the phase and theamplitude of the signals corresponding to the Foucault currents.

In a general manner, the device used for measuring the thickness of thecladding by Foucault currents comprises:

a checking head containing the Foucault-current probes and ensuring thepositioning of these probes on the tube, and the precise guidance of thetube,

at least one Foucault-current probe fixed on the checking head,

a source of multi-frequency exciting sinusoidal current,

mechanical means for driving and accurate guidance of the tubes past thechecking head,

highly accurate means for checking the linear advance of the tubes andfor measuring their axial-position, and

means for the acquisition and the data-processing of the Foucaultcurrent measurements carried out.

The obtention of an accurate value for the thickness e_(p), measured byFoucault currents and measurement of the passage time of a longitudinalultrasonic wave propagating in the total thickness of the jacket in adirection perpendicular to the surface, as shown in FIGS. 2A and 2B,makes it possible to obtain an accurate value for the total thickness ofthe Jacket.

This total thickness of the Jacket e_(g) is given by the formula e_(g)=e_(p) +(δt-e_(p) /V_(p))×V_(a), in which e_(p) represents the thicknessof the cladding measured by Foucault currents, V_(p) the speed of thelongitudinal ultrasonic waves in the cladding material, V_(a) the speedof the longitudinal ultrasonic waves in the material of the core of thetube, and δt the propagation time of the ultrasonic wave in the totalthickness of the jacket.

In this expression, e_(p) /V_(p) represents the passage time of theultrasonic wave in the cladding material, (δt-e_(p) /V_(p)) representsthe passage time of the ultrasonic wave in the core of the tube,(δt-e_(p) /V_(p))×V_(a) represents the thickness of the core, for anaxial position of the tube which is perfectly determined by virtue ofthe means for checking and measuring the axial position.

This calculation is, of course, only valid in the case where the speedsV_(p) and V_(a) are sufficiently different give rise to significanterrors during the measurement and calculation of the thickness of thetube.

The invention also permits the detection of flaws in cohesion at theinterface between the cladding and the core of the tube.

The flaws in cohesion are plane, of negligible thickness and parallel tothe surface of the tube.

It would therefore be very difficult to detect these flaws by Foucaultcurrents.

An ultrasonic detection technique is therefore better suited, althoughthe very small depth of the flaw beneath the surface of the tubecorresponding to the thickness of the cladding layer (between 80 and 100μm) constitutes a difficulty in detection of the flaws in cohesion atthe interface.

It is possible to use techniques for detection by the reflection ofultrasonic waves which are known per se and which are represented inFIGS. 4A, 5A and 6A and on the corresponding oscillograms of FIGS. 4B,5B and 6B.

The chief disadvantage of these reflection detection techniques lies inthe need to use ultrasonic waves at a very high frequency, for exampleat a frequency greater than 100 MHz, which corresponds to wave lengthsin the zirconium of less than 50 μm.

According to a first reflection detection technique, represented inFIGS. 4A and 4B, ultrasonic waves are emitted in substantially radialdirections with respect to the tube, in other words with a substantiallynormal incidence.

In FIG. 4A, an ultrasonic beam 21 has been shown, reflected on the outersurface of the tube, an ultrasonic beam 22 reflected on a flaw 20situated at the interface 4 between the cladding layer 3 and the core 2of the tube, and a beam 23 reflected on the inner surface of the tube,the corresponding echoes 24, 25 and 26 being shown in FIG. 4B.

The echo signal 26 reflected by the inner surface of the tube has asmaller amplitude than the signal 24 reflected by the outer surface ofthe tube. The time lag between these two echoes corresponds to twice thepassage time of the ultrasonic waves in the thickness of the tube.

The echo signal 25 corresponding to a reflection on a flaw 20 at theinterface 4 has a smaller amplitude and a very small time lag comparedwith the signal reflected on the outer surface of the tube because ofthe very small thickness of the cladding layer 3.

This first method of detection is therefore limited by the fact that theflaw is very close to the outer surface of the tube, and hence by thefact that the corresponding echo 25 can be mixed with the echo 24 whichhas a large time width due to the effect of the electronic amplificationof the ultrasonic signal.

A second method, illustrated by FIGS. 5A and 5B, consists in using abeam of ultrasonic waves 27 with oblique incidence so that this beam isfirst reflected by the inner surface of the tube,,then by the flaw 28 atthe interface and again by the inner surface of the tube.

In this case, the echo 29 corresponding to the reflection on the flaw 28after an initial reflection on the inner surface of the tube, followedby a second reflection on the inner surface of the tube, has aconsiderable time lag compared with the echo 24.

Similarly, the echo 29 and the immediately following echo 29 reflectedby the inner surface of the tube have a small, equivalent amplitude andtime width and can therefore be separated easily.

This technique can, however, be difficult to implement depending on thenature of the flaw and insofar as it must be carried out with obliqueincidence.

It may also be necessary to use an ultrasonic transducer with a separateemitter and receiver.

A third measuring method is illustrated by FIGS. 6A and 6B.

The checking is carried out from the inside of the tube and theultrasonic beam is emitted with normal incidence so as to obtain adirect reflection on the flaw 30.

The echo 31 corresponding to the reflection on the flaw 20 has a smalleramplitude and a large time lag compared with the signal reflected by theinner surface of the tube.

Similarly, this echo 31 and the immediately following echo 31' resultingfrom the reflection on the outer surface of the tube have small,equivalent amplitudes and time widths and can therefore be separatedeasily.

However, this detection method is difficult to implement in anindustrial context, insofar as the checking must be effected from theinside of a tube of small diameter and of great length.

It is thus difficult to obtain checking rates which are sufficient foruse of the method on an industrial scale.

Furthermore, the use of ultrasonic waves with very high frequencies hasdisadvantages in the case of the use of the method in an industrialenvironment, insofar as this method is sensitive to electronicinterference.

FIGS. 7A, 7B, 8A and 8B illustrate a technique for detecting flaws incohesion at the interface between the cladding layer 3 and the core 4 ofa duplex tube 1, by transmission of an ultrasonic wave in the wall ofthe duplex tube constituting a jacket for a fuel rod, the ultrasonicwave then being reflected on the inner surface of the tube, as can beseen in FIG. 7A which relates to a tube or part of a tube which has noflaw in cohesion.

In this case, the oscillogram shown in FIG. 7B has a bottom echo 36, theamplitude of which, although less than the amplitude of the input echo35, is considerable.

The application of the method to a sound material therefore results in avirtually integral transmission of the ultrasonic wave at the interfacebetween the cladding layer 3 and the core 2 of the tube. The reflectionat the interface 4 is, in fact, negligible insofar as the acousticimpedances of the materials constituting the cladding layer 3 and thecore 2 are very similar.

Where a flaw in cohesion 37 exists at the interface 4' between thecladding layer 3' and the core 2' of a duplex tube 1', as shown in FIG.8A, the ultrasonic wave emitted with a virtually normal incidence withrespect to the outer surface of the tube cannot be transmitted, or istransmitted only very partially, at the flaw in cohesion 37 situated atthe interface 4'.

The ultrasonic energy is dissipated by the successive reflections in thethickness of the cladding layer 3'.

A highly attentuated, or even non-existent, bottom echo 36' is thenobtained.

The input echo 35' is widened and represents the dissipation of theultrasonic energy by successive reflections in the cladding layer.

The method therefore makes it possible to distinguish very easily asound material from a material having flaws in the cohesion.

This transmission detection technique can be applied by using a beam ofultrasonic waves, the frequency of which is located at an intervalpermitting easier implementation of the detection method compared withthe reflection detection methods which have been described above.

This range of frequencies can lie, for example, between 10 and 20 MHz.Moreover, it is possible to use the ultrasonic transducer with normalincidence, which has advantages for the ease of implementation of themethod.

These conditions correspond in practice to those which are currentlyused in the case of checking the thickness of the wall of a fuel rodjacket.

In FIG. 9, an ultrasonic transducer, or sensor 40, has been shown whichmakes it possible to detect flaws in cohesion at the interface of aduplex tube 1.

The sensor 40 is designed as to obtain an optimized focusing of theultrasonic beam 41.

Since the flaws in cohesion at the interface of the duplex tube 1 areflaws which are elongated in the direction parallel to the axis of thetube and have a surface parallel to the surface of the tube, it isdesired to obtain a focal spot 42 of oblong shape, the longitudinal axisof which extends accurately in a direction parallel to the axis of thetube. The surface 43 of the focusing lens of the sensor has the shape ofa cylindrical sector, and the optimum adjustment of the focal spot isobtained by adjusting the orientation of the sensor so that the bottomecho (36 in FIG. 7B) has a maximum amplitude.

Furthermore, the sensor must have a wide pass band, which is obtained byhigh damping. Very narrow echoes are thus obtained and, moreover, theinput echo (35 in FIG. 7B) is clearly separated from the bottom echo (36in FIG. 7B). A better display of the time widening of the input echo(echo 35' in FIG. 8B) upon passage over a flaw in cohesion such as theflaw 37 (FIG. 8A) is also obtained.

The sensor 40 is mounted on a mechanical displacement assembly (notshown), which makes it possible to effect a fine adjustment of thefocusing of the sensor, of the alignment of the focal spot with respectto the axis of the tube, of the height of the coupling liquid such aswater, in other words the distance between the sensor and the tube, andof the incidence of the beam, and, to achieve an accurate guidance ofthe tube as it passes by in the direction of its axis beneath theultrasonic sensor 40.

The invention, in its various embodiments, therefore makes it possibleto check simply, quickly and accurately the thickness and the cohesionof the interface of a duplex tube by using simultaneously ultrasonicchecking techniques and Foucault-current checking techniques.

The implementation of the device according to the invention can easilybe achieved industrially, on a very large number of tubes of greatlength and of small diameter.

It is possible to use ranges of frequencies of the ultrasonic waveswhich are different from those which have been mentioned and transducershaving a form, a structure and dimensions which are adapted to the tubesto be checked. These transducers or sensors can be associated withmechanical adjustment means of any type.

The tube can be displaced in its longitudinal direction with respect tothe sensor by guide means and drive means of any type.

The position of the tube and on the zone being checked can be determinedaccurately by any suitable means.

It is likewise clear that devices can be used for measuring thethickness of the cladding layer by Foucault currents of a type differentfrom those which have been described.

The processing modules and the data-processing means associated with theultrasonic checking sensor and with the Foucault-current measuring meanscan consist of conventional components which digitize and process thesignals, calculate the thickness, display the results in any form andindicate the presence of flaws in the form of easily recognizablemessages.

Lastly, the invention applies to the checking of any duplex tube used asa jacket element for fuel rods of assemblies for nuclear reactors or inany other fields of industry.

Similarly, these types of checking can be applied even more easily tolarger tube diameters and thicknesses; the upper limit is fixed by theFoucault-current technique for measuring the thickness of the cladding,and this limit thickness is generally approximately 2 mm in the case ofthe abovementioned zirconium alloys.

I claim:
 1. Device for checking a thickness and cohesion of an interfaceof a duplex tube comprising a tubular core made from a first alloycovered with a cladding layer made from a second alloy, a base metal ofsaid first alloy, a base metal of said second alloy being identical to abase metal of said first alloy, said device comprising(a) at least oneultrasonic transducer disposed for emitting ultrasonic waves insubstantially radial directions with respect to said tube, from outsideto inside said tube in said cladding layer and said core of said tube;(b) detection and measuring means for detecting reflected ultrasonicwaves reflected by surfaces of said tube and for determining propagationtimes and amplitudes and shape of the reflected ultrasonic waves; (c) atleast one indication coil connected to supply exciting current to asource of multi-frequency alternating current; (d) means for measuringat least one of a phase and an amplitude of eddy currents induced insaid tube by said induction coil; (e) calculating means for calculatinga thickness of said cladding layer from one of said phase and amplitudeof eddy currents induced in said tube; (f) calculating means forcalculating a total thickness of said tube from the measurements ofpropagation times of said ultrasonic waves and the thickness of saidcladding layer; and (g) means for analyzing an amplitude and shape ofthe reflected ultrasonic waves.
 2. Device according to claim 1, whereinsaid exciting coil has turns encircling the tube and arrangedsuccessively in an axial direction of said tube.
 3. Device according toclaim 1, wherein said coil has an axis extending substantially radiallyof said tube.
 4. Device according to claim 1, comprising a plurality ofcoils having winding axes extending radially of said tube, said coilsbeing fixed in spaced circumferential positions on a support encirclingsaid tube.
 5. Device according to claim 1, comprising mechanical meansassociated with said tube to ensure its axial displacement with respectto said coil.
 6. Device according to claim 1, wherein said transducercomprises a focusing lens in the form of a portion of a cylinder, inorder to focus a beam of ultrasonic waves at an oblong focal spot havinga longitudinal axis parallel to the axis of said tube.